Research articleStructure of [60]fullerene with a mobile lithium cation inside
Abstract
The structure of crystalline [60]fullerene with a lithium cation inside (Li+@C60) was determined by synchrotron radiation X-ray diffraction measurements to understand the electrostatic and thermal properties of the encapsulated Li+ cation. Although the C60 cages show severe orientation disorder in [Li+@C60](TFPB−)·C4H10O and [Li+@C60](TFSI−)·CH2Cl2, the Li+ cations are rather ordered at specific positions by electrostatic interactions with coordinated anions outside the C60 cage. The Li+@C60 molecules in [Li+@C60](ClO4−) with a rock-salt-type cubic structure are fully disordered with almost uniform spherical shell charge densities even at 100 K by octahedral coordination of ClO4− tetrahedra and show no orientation ordering, unlike [Li+@C60](PF6−) and pristine C60. Single-bonded (Li+@C60−)2 dimers in [Li+@C60−](NiOEP)⋅CH2Cl2 are thermally stable even at 400 K and form Li+–C bonds which are shorter than Li+–C bonds in [Li+@C60](PF6−) and suppress the rotational motion of the Li+ cations.
1. Introduction
Hollow spherical carbon molecules called fullerenes or buckyballs with a molecular formula of C2n (n ≥ 30) can contain various atoms and molecules [1,2]. Endohedral fullerenes containing atoms and molecules with magnetic and electric moments have the potential to be used for molecular devices such as quantum bits [3,4], magnetic resonance imaging agents [5,6] and molecular switches [7,8]. [60]Fullerene C60 with a soccer-ball shape of 1 nm diameter is the most abundant fullerene in soot obtained by arc discharge of graphite electrodes and is valuable in applications such as solar cells [9,10]. Endohedral C60 with a lithium cation inside (Li+@C60) is efficiently obtained by lithium plasma bombardment with C60 vapour onto a target plate and is commercially available from Idea International Co., Ltd. [11–13]. It has been reported that Li+@C60 shows outstanding electron-accepting properties for applications such as photoelectrochemical solar cells [14–16] and non-volatile organic transistor-based memories [17].
Li+@C60 forms crystalline salts with anions such as SbCl6− and PF6− [11,18]. The PF6− salt, [Li+@C60](PF6−), has a rock-salt-type cubic structure and shows a fast response to an alternating electric field by free rotational motion of the Li+ cation on a shell with a radius of 1.5 Å inside the C60 cage [18–20]. The free rotational motion of the Li+ cation is suppressed so that it is localized at two off-centre equivalent positions on the threefold inversion axis of the cubic crystal below 100 K. The simultaneous occupation of the two positions at low temperature is attributed to a quantum tunnelling of the Li+ cation between the two positions with an interval of 2.7 Å. The tunnelling motion is also hindered below TC = 24 K with an abrupt decrease in the dielectric permittivity by antiferroelectric interactions among local electric dipole moments formed between the Li+ cations inside and the PF6− anions outside the C60 cages. The tunnelling motion and intermolecular interaction of the Li+ cation suggest that Li+@C60 has the potential to be used as a quantum bit in quantum computers using electric dipole moments.
The position and motion of a Li+ cation inside C60 are significantly affected by the coordinated anions and molecules outside the C60. For instance, the Li+ cation in the SbCl6− salt is localized at two adjoining off-centre positions at 370 K by asymmetric coordination of SbCl6− anions around an Li+@C60 molecule [11]. Such anion exchange effects should be investigated in more depth to understand the electrostatic responses of the encapsulated Li+ cations. Crystals of [Li+@C60](PF6−), [Li+@C60](SbCl6−) and [Li+@C60](TFPB−) (TFPB, tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) were previously obtained and structurally characterized [11,18,21]. Further structural study of [Li+@C60](TFPB−) is required, because the position and motion of the Li+ cation have not been revealed in the previous structural analysis [21]. The anions of the previously obtained crystals are all non-polar. Li+@C60 crystals combined with polar anions such as TFSI− (TFSI, bis(trifluoromethylsulfonyl)imide) should also be obtained and structurally characterized. PF6− and SbCl6− are small octahedral anions, whereas TFPB− is a large tetrahedral anion. It is worthwhile obtaining and investigating Li+@C60 crystals combined with small tetrahedral anions such as ClO4−. A crystal of dimerized Li+@C60− without anions was also obtained by electrochemical reduction [22]. The crystal is different from other dimerized endohedral metallofullerene crystals that consist of negatively charged endohedral metallofullerenes and positively charged donor molecules [23,24]. The Li+ cation in the dimerized Li+@C60− is localized near the carbon atom that is nearest to the carbon atom forming an inter-fullerene single C–C bond at 100 K. Temperature dependence of the position and motion of the localized Li+ cation in the dimerized Li+@C60− would provide us with essential information about the thermal stability of the (Li+@C60−)2 dimer and the Li+–C bond.
To understand the electrostatic and thermal properties of the mobile Li+ cation inside C60, we investigated the effects of the coordination structure and temperature on the position and motion of the encapsulated Li+ cations in Li+@C60 crystals by synchrotron radiation (SR) X-ray structure analysis in this study.
2. Material and methods
We performed the X-ray crystal structure analyses of [Li+@C60](TFPB−), [Li+@C60](TFSI−), [Li+@C60](ClO4−) and [Li+@C60−](NiOEP) (OEP, octaethylpolphyrine). The TFPB−, TFSI− and ClO4− salts were obtained by anion exchange of [Li+@C60](PF6−) [21]. Single crystals of the TFPB− and TFSI− salts and powder crystals of the ClO4− salt were obtained by vapour diffusion from solutions. The crystal structure of [Li+@C60](TFPB−) without solvent molecules at 123 K has already been reported [21]. A single crystal of [Li+@C60](TFPB−) containing diethyl ether solvent molecules was obtained in this study. Single crystals of [Li+@C60−](NiOEP) were obtained by electrochemical reduction in [Li+@C60](TFSI−) solution in the presence of NiOEP [22]. The crystal structure of [Li+@C60−](NiOEP) containing dichloromethane solvent molecules at 100 and 250 K has already been reported [22]. We report the crystal structure of [Li+@C60−](NiOEP) at 400 K and discuss the temperature dependence of the crystal structure in this study. The SR X-ray diffraction (XRD) measurements were performed at beamline BL02B1 of the SPring-8 large SR facility [25]. The crystal structures were determined by using SIR [26], SHELX [27] and SP [28]. The experimental conditions and crystallographic data are summarized in table 1 and crystallographic information files (CIFs) as the electronic supplementary material. The CIF deposition numbers at the Cambridge Crystallographic Data Centre (CCDC) are 1826722 for [Li+@C60](TFPB−), 1826723 for [Li+@C60](TFSI−) and 1826724 for [Li+@C60−](NiOEP).
| [Li+@C60](TFPB−) | [Li+@C60](TFSI−) | [Li+@C60](ClO4−) | [Li+@C60−](NiOEP) | |
|---|---|---|---|---|
| formula | LiC60·C32H12BF24· | LiC60·C2F6NO4S2· | LiC60·O4Cl | LiC60·NiC36N4H44· |
| C4H10O | CH2Cl2 | CH2Cl2 | ||
| formula weight | 1664.88 | 1092.62 | 827.03 | 1403.93 |
| crystal size (mm) | 0.25 × 0.12 × 0.11 | 0.30 × 0.08 × 0.04 | powder | 0.25 × 0.03 × 0.03 |
| temperature (K) | 260 | 150 | 30–450 | 400 |
| X-ray wavelength (Å) | 0.41324 | 0.70220 | 0.64898 | 0.41400 |
| crystal system | monoclinic | orthorhombic | cubic | monoclinic |
| space group | P21/c | Pnma | C2/m | |
| unit cell parameters | a = 17.1122(9) Å | a = 18.963(5) Å | a = 14.133(1) Å | a = 24.6791(13) Å |
| b = 20.3829(12) Å | b = 13.892(3) Å | V = 2822.9(6) Å3 | b = 14.9697(6) Å | |
| c = 19.8912(18) Å | c = 13.964(3) Å | (100 K) | c = 17.7061(15) Å | |
| β = 105.157(5)° | V = 3678.5(15) Å3 | β = 109.171(5)° | ||
| V = 6696.6(8) Å3 | V = 6178.5(7) Å3 | |||
| Z | 4 | 4 | 4 | 4 |
| no. of independent reflections | 13 115 (d > 0.80 Å) | 3684 (d > 0.78 Å) | 162 (d > 0.85 Å) | 8779 (d > 0.70 Å) |
| (1434 data points) | ||||
| no. of parameters | 1459 | 433 | 55 | 657 |
| reliable factors | R = 0.125 (|F| > 4σ) | R = 0.098 (|F| > 4σ) | Rwp = 0.036 (100 K) | R = 0.067 (|F| > 4σ) |
| Rw = 0.363 (|F| > 4σ) | Rw = 0.312 (|F| > 4σ) | RI = 0.124 (100 K) | Rw = 0.200 (|F| > 4σ) |
3. Results and discussion
3.1. [Li+@C60](TFPB−)·C4H10O
Figure 1 shows the crystal structure of [Li+@C60](TFPB−) containing diethyl ether (C4H10O) solvent molecules at 260 K. The crystal shows a structural phase transition with a non-merohedral twinning around 250 K. The crystal structure above the phase transition temperature was determined in this study. The monoclinic lattice constants (a = 17.11 Å, b = 20.38 Å, c = 19.89 Å, β = 105.16°) and molecular arrangement are rather different from those of a solvent-free crystal (a = 13.40 Å, b = 26.03 Å, c = 17.03 Å, β = 90.52°) reported in [21]. The volume of the unit cell containing four [Li+@C60](TFPB−) ion pairs for the solvent-containing crystal (6697 Å3) is larger than that for the solvent-free crystal (5939 Å3). As a result, the coordination structures around the Li+@C60 are different between the solvent-free and solvent-containing crystals. The C60 centres are in the general positions of monoclinic structures in both crystals. The Li+@C60 molecule in the solvent-free crystal is coordinated by six TFPB− anions with an octahedral arrangement. The distance from the C60 centre to the nearest central boron atom of TFPB− is 8.1 Å. The distance from the C60 centre to the nearest C60 centre is 11.4 Å. The crystal structure of the solvent-containing [Li+@C60](TFPB−) consists of one-dimensional Li+@C60 arrays and TFPB− arrays along the c-axis, as shown in figure 1b,c. The distance from the C60 centres to the nearest central boron atom of TFPB− is 8.7 Å. The distance from the C60 centre to the nearest two C60 centres along the c-axis is 10.0 Å. Therefore, Li+@C60 to TFPB− distances are increased and Li+@C60 to Li+@C60 distances are decreased by intercalation of C4H10O solvent molecules.
Figure 1. Crystal structure of [Li+@C60](TFPB−)·C4H10O at 260 K. (a) Structure with thermal ellipsoids at the 50% probability level viewed along the c-axis. Hydrogen atoms are drawn as small spheres. Disordered structures are omitted. (b) Molecular arrangement viewed along the c-axis. (c) Molecular arrangement viewed along the b-axis. (d) Disordered structure of the C60 cage with the electron charge-density surface at 1.5 e/Å3 obtained by the maximum entropy method. A pentagon of the major orientation and four hexagons of the minor orientations are overlapping in the structure. (e) Structure of Li+@C60 with thermal ellipsoids at the 50% probability level viewed perpendicular to a hexagon near the violet encapsulated Li+ cation.
The differences in coordination structure around Li+@C60 between the solvent-free and solvent-containing crystals affect the rotational motion of the C60 cages. Fullerene molecules often show rotational motion and orientation disorder in crystals. For instance, C60 cages in [Li+@C60](PF6−) and pristine C60 crystals show free rotational motion at high temperature [18,29]. The free rotational motions are stopped at low temperature by phase transitions at 370 K in [Li+@C60](PF6−) and 260 K in pristine C60. Although the C60 orientation is perfectly ordered in the low temperature [Li+@C60](PF6−) [18], orientation disorder with two molecular orientations remains in the low-temperature pristine C60 [30,31]. This suggests that rotational motion of the C60 cages is hindered by electrostatic interactions between the cationic Li+@C60 and coordinated anions. C60 cages in the reported crystal structure of the solvent-free [Li+@C60](TFPB−) show no orientation disorder [21]. The electrostatic interactions between Li+@C60 and TFPB− should be weakened by the increase in Li+@C60 to TFPB− distances by intercalation of C4H10O solvent molecules. As a result, C60 cages in the solvent-containing [Li+@C60](TFPB−) crystal show a severe orientation disorder with five molecular orientations, as shown in figure 1d. The site occupancies for the five C60 orientations are 0.372(5), 0.253(4), 0.155(4), 0.132(3) and 0.124(4).
A weak electron charge-density peak for an Li+ cation was observed inside the disordered C60 cage of the solvent-containing [Li+@C60](TFPB−) at 260 K. The Li+ position at (x, y, z) = (0.235, 0.758, 0.151) is close to the gravity centre at (0.228, 0.741, 0.147) for the central boron atoms of the six TFPB− anions adjacent to the Li+@C60 with an interval of 0.13 Å, and, hence, the Li+ position is electrostatically consistent with the anion arrangement. The Li+ position is beneath the centre of a hexagon of the C60 with the major orientation, as shown in figure 1e. The average Li+–C distance is 2.35(10) Å, which agrees with the value for [Li+@C60](PF6−) at low temperature [18,19]. Assuming that the site occupancy for the Li+ position is the same as that of the major C60 orientation (0.372(5)), the isotropic atomic displacement parameter of the Li+ is refined to 0.24(3) Å2, and the remaining fraction of the Li+ cation would occupy other positions inside the C60 by a positional disorder. The Li+ position in the solvent-free [Li+@C60](TFPB−) has not been determined even at 123 K [21]. The Li+ cation of cubic [Li@C60](PF6−) is freely rotating on a shell with a radius of 1.5 Å inside the C60 cage above 100 K [18–20]. Therefore, the Li+ cation is partially ordered in the solvent-containing [Li+@C60](TFPB−) by the asymmetric coordination of TFPB− anions and C4H10O molecules, as shown in figure 1b,c.
3.2. [Li+@C60](TFSI−)·CH2Cl2
Figure 2 shows the crystal structure of [Li+@C60](TFSI−) containing dichloromethane (CH2Cl2) solvent molecules at 150 K. The crystal shows a structural phase transition with a non-merohedral twinning around 130 K. The crystal structure above the phase transition temperature was determined in this study. The TFSI− anion and CH2Cl2 molecule are polar molecules, while the TFPB−, ClO4−, SbCl6− and PF6− anions are non-polar molecules. Permanent electric dipole moments of such polar molecules should electrostatically interact with the Li+ cations inside the C60 cages in crystals. However, the electric dipole moments of TFSI− and CH2Cl2 are cancelled by the antiferroelectric arrangements in the crystal structure with a non-polar space group of Pnma. If an Li+@C60 crystal containing polar anions with a polar space group was obtained, the crystal could have a macroscopic electric dipole moment and exhibit ferroelectricity due to a switching of the polar anions and motion of the Li+ cations. Ferroelectric crystals of C60 encapsulating a polar molecule have been predicted theoretically [32]. Actually, a cubic crystal of C60 with a polar water molecule inside (H2O@C60) shows an increase in the dielectric permittivity according to the Curie–Weiss law at low temperature, although no ferroelectric phase transition is observed down to 8 K [33].
Figure 2. Crystal structure of [Li+@C60](TFSI−)·CH2Cl2 at the 150 K. (a) Structure with thermal ellipsoids at the 50% probability level viewed along the b-axis. Hydrogen atoms are drawn as small spheres. Disordered structures are omitted. (b) Molecular arrangement viewed along the a-axis. (c) Molecular arrangement viewed along the c-axis. (d) Disordered structure of the C60 cage with the electron charge-density surface at 1.7 e/Å3 obtained by the maximum entropy method. Two hexagons of the major orientations and a pentagon of the minor orientation are overlapping in the structure. (e) Structure of Li+@C60 with thermal ellipsoids at the 50% probability level viewed perpendicular to a hexagon near the violet encapsulated Li+ cation.
The molecular arrangement of Li+@C60 molecules in the bc-plane shown in figure 2b is similar to that of [Li+@C60](TFPB−)·C4H10O in the ab-plane shown in figure 1b and the face-centred molecular arrangement in [Li+@C60](PF6−). The lattice constants for b = 13.89 Å and c = 13.96 Å for [Li+@C60](TFSI−)·CH2Cl2 at 150 K are much smaller than a = 17.11 Å and b = 20.38 Å for [Li+@C60](TFPB−)·C4H10O at 260 K, and comparable to the cubic lattice constant of a = 14.30 Å for [Li+@C60](PF6−) at 150 K. The monolayer lattice in the bc-plane of [Li+@C60](TFSI−)·CH2Cl2 is stacked along the a-axis with a shift to the c-axis. If the shift is 0, the Li+@C60 arrangement becomes similar to that in [Li+@C60](TFPB−)·C4H10O, as shown in figure 1b,c. If the shift is c/2, the Li+@C60 arrangement becomes similar to the face-centred molecular arrangement in [Li+@C60](PF6−). Therefore, the molecular arrangements of [Li+@C60](TFPB−)·C4H10O (figure 1b,c) and [Li+@C60](TFSI−)·CH2Cl2 (figure 2b,c) can be classified into deformed rock-salt-type structures.
C60 cages in [Li+@C60](TFSI−)·CH2Cl2 also show a severe orientation disorder, as shown in figure 2d. The C60 cage was modelled by an overlap of three partially occupied C60 cages with different orientations. The site occupancies for the three C60 orientations are 0.413(3), 0.413(3) and 0.174(5). The two major orientations are equivalent to the crystallographic mirror symmetry. The number of overlapping orientations is less than that in [Li+@C60](TFPB−)·C4H10O (figure 1d). The orientation disorder of C60 is strongly affected by the exchange of anion and solvent molecules and is suppressed at low temperature. Anion exchange from larger TFPB− to smaller TFSI− should increase the van der Waals inter-fullerene interactions and electrostatic Li+–anion interactions. The measurement temperature of [Li+@C60](TFSI−)·CH2Cl2 (150 K) is also lower than that of [Li+@C60](TFPB−)·C4H10O (260 K). These are reasons for the suppression of C60 orientation disorder in [Li+@C60](TFSI−)·CH2Cl2. On the other hand, C60 cages in the solvent-free [Li+@C60](TFPB−) show no orientation disorder [21]. It is expected that C60 orientations are also ordered in solvent-free [Li+@C60](TFSI−).
The Li+ cation in [Li+@C60](TFSI−)·CH2Cl2 also locates beneath the centre of a hexagon of the C60 with the major orientation, as shown in figure 1e. The average Li+–C distance of 2.42(7) Å is consistent with that in [Li+@C60](TFPB−)·C4H10O. The Li+ position at (0.803, 0.750, 0.040) is close to the gravity centre at (0.763, 0.750, 0.083) for central nitrogen atoms of the six TFSI− anions adjacent to the Li+@C60 with an interval of 0.98 Å, and, hence, the Li+ position is electrostatically consistent with the anion arrangement. The flat thermal ellipsoid for the Li+ cation with a site occupancy of 1.0 suggests a large librational motion of the Li+ cation. The free rotational motion and positional disorder of the Li+ cation in [Li+@C60](PF6−) and solvent-free and solvent-containing [Li+@C60](TFPB−) will be suppressed by anion exchange from non-polar PF6− and TFPB− anions to polar TFSI− anions.
3.3. [Li+@C60](ClO4−)
Figure 3a,b shows the powder XRD pattern of [Li+@C60](ClO4−) at 100 K (X-ray wavelength: 0.649 Å) with a fitting result by the Rietveld refinement. A face-centred-cubic (fcc) structure model similar to the crystal structure of [Li+@C60](PF6−) above 370 K was used in the refinement. [Li+@C60](PF6−) and pristine C60 undergo a phase transition from the fcc structure (space group: ) to the low-temperature simple-cubic structure (space group: ) at 370 K and 260 K, respectively [18,29]. Surprisingly, [Li+@C60](ClO4−) has an fcc structure even at 100 K. The cubic lattice constant of [Li+@C60](ClO4−) is a = 14.13 Å at 100 K, which is smaller than that of [Li+@C60](PF6−) at 100 K (a = 14.28 Å) and larger than that of pristine C60 at 100 K (a = 14.06 Å) [18,31]. This relationship is consistent with the fact that a ClO4− anion is smaller than a PF6− anion.
Figure 3. (a,b) Powder XRD pattern of [Li+@C60](ClO4−) at 100 K (X-ray wavelength: 0.649 Å) with a fitting result by the Rietveld refinement. Observed and calculated intensities are plotted by red crosses and black lines, respectively. Deviations between observed and calculated intensities are plotted by blue lines. Low and high 2θ angle regions are shown separately in (a) and (b), respectively. (c) Crystal structure model of [Li+@C60](ClO4−) viewed along the threefold inversion axis. Grey and red spheres are uniform C60 shells and oxygen atoms of disordered ClO4− anions, respectively. (d) Temperature dependence of the cubic lattice constant of [Li+@C60](ClO4−) from 30 to 450 K.
Figure 3c shows the crystal structure model used in the powder pattern fitting. The space group is . The uniform spherical C60 shell with a radius of 3.55 Å centred at 0, 0, 0 and the disordered ClO4− anion centred at 1/2, 1/2, 1/2 form a rock-salt-type cubic structure. The disordered ClO4− anion was modelled by a Cl atom at 1/2, 1/2, 1/2 and partially occupied O atoms at 1/2 ± 0.059, 1/2 ± 0.059, 1/2 ± 0.059 with a site occupancy of 1/2. The disordered structure is given by orthogonal overlap of two ClO4− tetrahedra with a Cl–O distance of 1.44 Å. The powder pattern was fitted by using the simple structure model with acceptable reliable factors (Rwp = 0.036, RI = 0.124). However, the reliable Li+ position could not be determined due to the multi-site occupation or free rotational motion. The remaining deviations between the observed and calculated intensities in figure 3a,b are mainly due to non-uniformity of the electron charge densities on the C60 shell.
Figure 3d shows the temperature dependence of the cubic lattice constant of [Li+@C60](ClO4−) from 30 to 450 K. No phase transition was observed in the temperature range. A ternary alkali-doped fulleride Li2CsC60 also shows no phase transition and has an fcc structure from 50 to 300 K [34]. Disordered occupation of small Li+ and large Cs+ cations at tetrahedral voids at 1/4, 1/4, 1/4 and octahedral voids at 1/2, 1/2, 1/2 would hinder the orientation ordering of C60 at low temperature in Li2CsC60. By contrast, tetrahedral ClO4− anions occupy the octahedral voids in [Li+@C60](ClO4−), as shown in figure 3c. The mismatch between the molecular symmetry and the site symmetry would hinder the orientation ordering of ClO4− and C60 at low temperature in [Li@C60](ClO4−). [Li+@C60](PF6−), in which octahedral PF6− anions occupy the octahedral voids, shows perfect orientation ordering of C60 below 370 K [18].
3.4. [Li+@C60−](NiOEP)·CH2Cl2
Figure 4 shows the temperature dependence of the molecular structure of the single-bonded (Li+@C60−)2 dimer in [Li+@C60−](NiOEP)⋅CH2Cl2. The (Li+@C60−)2 dimer has a disordered structure by a ratchet-type rotation along the inter-fullerene single C–C bond with a rotation angle of about 39° above the phase transition temperature around 250 K [22]. The disordered structures at 250 and 400 K are omitted in the figure. (C60−)2 dimers in complex crystals of C60 with donor and solvent molecules dissociate above 160–220 K [35,36]. By contrast, the (Li+@C60−)2 dimers in [Li+@C60−](NiOEP)⋅CH2Cl2 are thermally stable even at 400 K, as shown in figure 4c. It is suggested that the inter-fullerene single C–C bond is stabilized by the encapsulation of Li+ cations. Low stability of (C60−)2 dimers is due to the strong repulsion of two negative charges within the dimer. The Li+ cation inside the cage which compensates this negative charge contributes essential stabilization of (Li+@C60−)2 dimers.
Figure 4. Molecular structure of the (Li+@C60−)2 dimer in [Li+@C60−](NiOEP)⋅CH2Cl2 at (a,d) 100, (b,e) 250 and (c,f) 400 K. (a–c) View perpendicular to the inter-fullerene single C1–C1 bond. (d–f) View perpendicular to a pentagon near the violet encapsulated Li+ cation. The thermal ellipsoids are drawn at the 50% probability level. Disordered structures at 250 and 400 K and coordinated NiOEP and CH2Cl2 molecules are omitted.
The encapsulated Li+ cation is localized with a site occupancy of 1.0 near the carbon atom (C2) nearest to the carbon atom (C1) forming the inter-fullerene single C–C bond (figure 4a). The Li+–C2 distance is 2.21(1), 2.31(2) and 2.36(6) Å at 100, 250 and 400 K, respectively. The value at 100 K is obviously shorter than the Li+–C distance of 2.37(1) Å in [Li+@C60](PF6−) at 40 K [19]. It is also noted that the Li+ cation in the (Li+@C60−)2 dimer locates near the centre of a pentagon involving C2, as shown in figure 4d,e, while the Li+ cation in [Li+@C60](PF6−) locates near the centre of the hexagons [18,19]. The formation of the shorter Li+–C bond contributes to stabilization of the (Li+@C60−)2 dimer. The inter-fullerene C1–C1 distance shows almost no temperature dependence, which are 1.59(1), 1.59(1) and 1.60(1) Å at 100, 250 and 400 K, respectively.
The thermal ellipsoid of the Li+ cation is unusually large perpendicular to the radial direction from the C60 centre at 400 K (figure 4c,f). The equivalent atomic displacement parameters of the Li+ cation and C2 atom are 0.8(1) and 0.077(2) Å2, respectively. The thermally induced large librational motion of the Li+ cation in the (Li+@C60−)2 dimer is basically consistent with the free rotational motion of the Li+ cation in [Li+@C60](PF6−) above 100 K. The Li+ cation bonded to the C2 atom cannot rotate freely in the (Li+@C60−)2 dimer even at 400 K.
4. Conclusion
We determined structures of cationic Li+@C60 monomers and neutral (Li+@C60−)2 dimers in crystals by SR XRD measurements to understand the electrostatic and thermal properties of the encapsulated Li+ cation. [Li+@C60](TFPB−)·C4H10O and [Li+@C60](TFSI−)·CH2Cl2 involve severe orientation disorder of the C60 cages at low temperature. However, the Li+ cations are rather ordered at specific positions by electrostatic interactions from non-polar TFPB− or polar TFSI− anions, which asymmetrically coordinate to the C60 cage. [Li+@C60](ClO4−) has a rock-salt-type cubic structure similar to the structure of [Li+@C60](PF6−). The Li+@C60 molecules in [Li+@C60](ClO4−) are fully disordered with almost uniform spherical shell charge densities by octahedral coordination of ClO4− tetrahedra even at 100 K, and show no orientation ordering or structural phase transition unlike [Li+@C60](PF6−) and pristine C60. Single-bonded (Li+@C60−)2 dimers in [Li+@C60−](NiOEP)⋅CH2Cl2 are stable even at 400 K, while (C60−)2 dimers in a complex of C60 dissociate. The formation of Li+–C bonds, which are shorter and thermally more stable than Li+–C bonds in [Li+@C60](PF6−), stabilizes the inter-fullerene single C–C bond and suppresses the free rotational motion of the Li+ cation. The variety of structures of Li+@C60 revealed in this study proves the controllability of the position and motion of the encapsulated Li+ cation from outside the C60 cage, which would be valuable in future applications of Li+@C60 as molecular devices. This study shows that the spherical Li+@C60 cation forms various ionic crystals with common inorganic and organic anions, and the neutral Li+@C60− consisting of a positive Li+ core and a negative C60− cage forms thermally stable molecular crystals of covalently bonded (Li+@C60−)2 dimers. The results are essential and important to support the strong superatomic character of Li+@C60.
Data accessibility
The CIFs are available as the electronic supplementary material and from the Cambridge Crystallographic Data Centre (CCDC) (www.ccdc.cam.ac.uk/structures) with deposition numbers 1826722, 1826723 and 1826724.
Authors' contributions
S.A. carried out the SR XRD experiments and crystal structure analyses, and wrote the manuscript. K.M. carried out the crystallization and crystal structure analysis. H.U. and H.O. carried out the anion exchange, electrochemical reduction and crystallization. Y.M. and K.K. supervised and conducted the sample preparations and chemical analyses. All authors gave final approval for publication.
Competing interests
We declare we have no competing interests.
Funding
This work was financially supported by The Mitsubishi Foundation and The Murata Science Foundation.
Acknowledgements
The synchrotron radiation experiments were performed at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (proposal nos. 2013A0100, 2013B0100, 2015A0100, 2017A1206 and 2017A1739). S.A. thanks Mr Y. Yokoi, Ms Y. Nakahira and Dr K. Sugimoto for their help with the experiments.
